Conventional photomasks commonly consist of a substrate, e.g., fused silica plate, having an opaque chrome film. Photomasks are produced from these blanks by providing a desired pattern of open areas in the film. In use, light is optically projected through the open areas of the photomask onto the surface of a light sensitive substrate, such as a photoresist or photopolymer-coated semiconductor wafer. Currently, photomasks are illuminated with visible or ultraviolet light. A fundamental limitation of optical imaging is that line widths of the order of the wavelength of the illuminating light are diffraction limited. In other words, light having a wavelength of the same order of magnitude as the desired optical image will be diffracted, and the projected image will be wider than the desired image.
The electronics industry seeks to extend optical lithography for manufacture of high density integrated circuits to critical dimensions of 0.25 .mu.m and smaller. To achieve this objective, lithographic photomasks will need to work with short wavelength , i.e., &lt;400 nm, light. Three wavelengths targeted for future optical lithography are 248 nm (KrF laser wavelength), 193 nm (ArF laser wavelength), and 157 nm (F.sub.2 laser wavelength). A phase shift photomask enhances the patterned contrast of small circuit features by destructive optical interference of the light projected through the open areas in the film with light projected through the film on the substrate.
In this effort to extend optical lithography, a variety of phase-shifting photomasks have been developed for ultraviolet and visible light ranges. See, e.g., B. J. Lin, The Attenuating Phase-Shifting Mask, Solid State Technology, pp. 43-47, January, 1992. Attenuated phase-shifting ("APS") photomasks employ an absorptive, partially transmitting, phase shifter in place of the conventional opaque chromium part of the patterned film. The absolute transmission of the absorptive phase shifter is adjusted to less than about 0.25 to prevent creation of ghost lines in the image on the photoresist coated semiconducor wafer. However, not all materials can both phase shift and absorb by the desired amount. Consequently, a multilayered structure consisting of materials of differing absorptive and phase shifting properties may be required in some cases.
For example, a commercially available APS mask, commonly referred to as a "leaky chrome" type of APS, utilizes a chromium oxycarbonitride composition film having a variable, graded composition comprising a Cr--N rich composition at the substrate-film interface to a Cr--O rich composition at the film-air interface. The Cr--O rich composition also serves as an anti-reflective coating. While this APS masks provides a degree of phase shifting, a further procedure, such as reactive ion etching of the fused silica substrate, or the addition of a second material, such as spin-on-glass, as the phase shifter is necessary to achieve the desired 180.degree. phase shift, or a odd integer multiple thereof, at the selected lithographic wavelength.
The concept of a phase shift photomask that attenuates light and changes its phase in a single film material so as to produce an attenuating embedded phase shift ("AES") photomask, is disclosed in U.S. Pat. No. 4,890,309. Known AES photomasks fall mainly into two categories: (1) Cr-based photomasks containing Cr, Cr-oxide, Cr-carbide, Cr-nitride, Cr-fluoride or combinations thereof (see, e.g., U.S. Pat. No. 5,459,002 and U.S. Pat. No. 5,415,953); and (2) SiO.sub.2 - or Si.sub.3 N.sub.4 -based photomasks, containing SiO.sub.2 or Si.sub.3 N.sub.4 together with a predominantly opaque material, such as MoN or MoSi.sub.2. Commonly the latter materials are referred to generically as `MoSiON`. In addition, AES photomasks comprising hydrogenated amorphous carbon layers, tantalum and its compounds with a layer of Cr metal, or one or more layers composed of a hafnium compound, are also known in the art.
Polymeric materials are known to be useful as phase shift materials in APS masks. For example, organic containing polymers deposited by spin coating are known to serve as the spin on glass phase shift material in a leaky chrome type of APS mask discussed above. These attenuating phase shift photomasks are not considered an attenuating embedded phase shifter photomasks because the polymeric material, be it a silicate based spin on glass or an organic containing polymeric material, which provides the necessary degree phase shift of the transmitted light, is a separate material from the material which predominantly determines the level of attenuation of the photomask. While these polymer containing APS masks find use in research on phase shift lithography, the disadvantages of producing phase shift photomasks using separate attenuator and phase shifter materials restricts their use in large scale commercial production. Accordingly, the AES masks of choice for commercial use are those based on vapor deposited inorganic materials, such as those described, for example, in U.S. Pat. No. 5,459,002 and U.S. Pat. No. 5,415,953 have found wide spread use.
U.S. Pat. No. 5,726,247 discloses a fluoropolymer nonocomposite material.